Objectives of the so-called XL (=existing light) class, designed to take indoor pictures without additional illumination, generally have relative apertures of about 1:1.3 or larger. In many instances, as described for example in commonly owned U.S. Pat. No. 3,989,349, the rear lens group of such an objective consists of five air-spaced lens members. In an objective of the type here contemplated, e.g. as known from U.S. Pat. Nos. 3,044,355, 3,074,317 and 3,580,662 as well as German published specification No. 2,526,847, the diaphragm is usually located immediately in front of this rear lens group and is preceded by the shiftable positive third component. Obviously, this third component as well as the lenses of the rear group must have diameters compatible with the overall relative aperture as determined by the interposed diaphragm; thus, the third component must be a fairly large lens member though generally smaller than the member or members of the front component whose diameter determines the field of view.
Unless the third component is a lens member of large individual focal length f.sub.3, its spherical aberrations create problems which heretofore were resolved by designing this lens member as a cemented doublet. An excessive increase of focal length f.sub.3 generally leads to objectionably large dimensions for the varifocal front group or necessitates a reduction in the varifocal ratio if the desired large relative aperture is to be preserved. As a practical rule, the individual focal length f.sub.2 of the second (negative) component should be on the same order of magnitude as the median overall focal length f.sub.med of the entire objective, i.e. the focal length existing at the inversion point of the path of the third component which then lies in a position farthest from the rear lens group. With modern objectives, the preferred absolute value of f.sub.2 ranges between about 0.7 and 1.5 times f.sub.med.